The present invention relates to a device for use on so-called high frequency track circuits, such circuits being extensively used to determine the presence of a railroad vehicle or the like within a section of track defined by the positioning of a transmitting unit and a receiving unit.
A great number and variety of track circuits have been developed over the past 30 years or so, and the principal purpose of such circuits is generally to detect the presence of a railroad vehicle effectively operating as a shunt across the rails, thereby to indicate the presence of such vehicle. The end to be realized is that the shunting caused by the presence of the vehicle will result in the production of block signals useful for indicating to other vehicles that a particular block or zone is occupied.
Some of the aforenoted track circuits are DC in nature and often involve insulated joints. One can have relatively long DC track circuits and even with a poorly shunting vehicle, a relay which normally is held up at one end by a power source feeding the track circuit from the other end, will be de-energized or drop away, thereby providing detection of such vehicle even under the most unfavorable conditions. However, with jointless track circuits, the track rail is continuous and termination of the track circuit is accomplished by means of a shunt across the rails at the extreme ends of the circuit.
Despite certain advantages to DC track circuits, the most common form of present day track circuit is one operating with AC. Using alternating current, however, introduces the factor of inductive rail impedance which increases as the frequency of the input current increases.
In order to provide complete background for the subject matter of the present invention, reference may be made to certain prior art control systems and associated track circuits in U.S. Pat. Nos. 1,852,571; 3,663,809; 3,670,161; 3,715,669.
In particular, the system described in U.S. Pat. No. 3,715,669 involves a receiver forming part of an overlay track circuit in which a frequency modulated signal having alternately occurring extreme side band components is communicated along a railroad right-of-way. Such system is subject to the influence of the presence of a railroad vehicle and the system comprises a tuner means responsive to the FM signals separating one side band component of the signal from the other, and polarizing means for generating distinctive signals of opposite polarity in response to the respective side band components. The receiver in the aforenoted system is coupled to rails by a directly connected series resonant circuit including a coil and a capacitor and a further coupling coil. Thus the receiver filters and amplifies a signal for transmission to the next stage of the system.
Whatever the particular components and features that may be utilized in a high frequency AC track circuit, the present invention is designed and constructed specifically to overcome a fundamental problem occurring in such context. More particularly, in the specific context of a pulse code modulated track circuit the invention is particularly advantageous in that it overcomes an infortuitous "filling in" by an undesired signal with reference to a generated pulse code pattern such that the track relay would be caused to drop even though a train is not on the predetermined section of track. This aspect will become clear as the description proceeds.
The particular problem arises due to the fact that harmonic frequencies result from the traction power used on the railroad right-of-way; thus, the traction power is frequently switched on and off in such a way that the harmonic frequencies which are generated are coupled from the third rail to the near running rail to a substantially higher degree than they are coupled from the third rail to the far running rail. The eventual result is that a circulating alternating current develops in the track circuit, recalling that the track circuit utilizes the same running rails. There is no problem with DC because the return thereof is split equally between the running rails.
Accordingly, it is a primary object of the present invention to overcome or prevent interference with the normal functioning of a pulse modulated or similar AC track circuit by reason of the adventitious coupling thereto of the harmonic frequencies of the traction power utilized for operating motors and the like. These harmonic frequencies in particular regard to pulse code modulated track circuits, function to fill in the gaps, so to speak, during the OFF periods of the normal pulse code signal and thus interfere with normal operation causing a remote track circuit to give an erroneous signal.
The above and other objects are achieved by a principal feature of the present invention which comprises a transducer coil arrangement which is so installed in respect to the affected track circuit that a cancellation effect is achieved in regard to the circulating current caused by the aforesaid undesired coupling of the traction power harmonic frequencies. Thus, the circulating current is effectively no longer present and its effects are obviated.
In accordance with such feature, a coil made up of several turns of wire is provided and is run close and parallel to the third rail and also close and parallel to the far running rail. By selecting a capacitor of the appropriate size and by placing the capacitor in circuit with the coil, the coil can be tuned to the harmonic frequency that is the source of the coupling problem. Moreover, by the use of a suitable jumper the particular number of turns required to offset the effect of the circulating current could be adjusted in the field. In other words, the particular magnitude or value of induced voltage can be selected by such appropriate selection of the number of turns.
In accordance with the coil arrangement of the present invention, the harmonic noise frequency generated in the third rail is magnetically coupled to the cancellation coil, that is, the transducer coil that is specially arranged as aforenoted. In turn, because of this particular arrangement, the coil also magnetically couples the same frequency into the far running rail. Consequently, the coupled signal is completely out of phase with the coupled signal present in the nearest rail. As an ultimate result the signal coupled from the cancellation coil cancels the effect of the circulating current which is causing the problem. It virtually completely reduces the in band noise seen by the receiver.
Other and further objects, advantages and features of the present invention will be understood by reference to the following specification in conjunction with the annexed drawing, wherein like parts have been give like numbers.